Sub fecundity (difficulty conceiving a pregnancy) is a pressing public health problem. Approximately 6.7 million (11%) U.S. women of reproductive age have impaired fecundity and 1.5 million are infertile. Moreover, women are delaying child-bearing to older ages. It is well-known that risk of sub fecundity and adverse reproductive outcomes increases with age. With more women attempting pregnancy at older ages, it is increasingly important to understand modifiable risk factors for adverse reproductive outcomes in this population. Small studies have reported detrimental effects of low vitamin D levels on reproduction, but large population-based human data are lacking. The objective of this project is to examine the role of vitamin D in human female reproductive function, among women over 30 years of age, with the ultimate goal of evaluating the potential of vitamin D as a modifiable risk factor for sub fecundity. To accomplish this objective we will first investigate predictors of baseline vitamin D and the accuracy of embryonic growth measurement (K99) and apply the results of this investigation to the assessment of vitamin D in reproduction (R00). This research will capitalize on an ongoing R01 study of fecundity, Time to Conceive. Time to Conceive is a prospective, cohort study of women attempting to become pregnant. During each menstrual cycle participants test their urine for ovulation. Women that conceive undergo an early pregnancy ultrasound. This proposal has three specific aims, 1) identify predictors of baseline vitamin D levels in a cohort of late reproductive age women (K99), 2) to identify factors that influence early embryonic growth and assess the bias produced by those factors in currently-used ultrasound dating methods (K99), and 3) to elucidate the role of vitamin D in female reproductive function by quantifying its associations with sub fecundity (as measured by time to pregnancy), anti-Mullerian hormone, follicle stimulating hormone, menstrual cycle length, and embryonic growth (R00). To achieve these aims the circulating metabolite of vitamin D, 25-hydroxyvitamin D (25(OH) D), will be measured in blood spots take early in a woman's attempt to become pregnant and again in blood spots drawn in early pregnancy (7-9 weeks gestation). This project will provide the first comprehensive examination of vitamin D and early reproduction in a healthy population-based study. This proposal fits well with the candidate's long-term research interest of examining the physiologic basis of infertility and identifying modifiable factors (including nutritional factors) that can improve the probability of conception for women. The completion of this project will provide pilot data for a subsequent R01 application when the candidate transitions to an independent academic position. Through this project the candidate will receive training in nutritional epidemiology and nutritional biochemistry.